For many years decorative laminates have been used as a surfacing material in residential and commercial structures wherein aesthetic effects are desired in combination with functional behavior such as wear, heat and stain resistance. Typical applications of said laminates are surfacing for walls, partitions, table tops, counter tops, furniture, doors and the like. Such decorative laminates generally are produced from a supporting base member, usually either a particleboard product or a plurality of resin impregnated core sheets usually composed of kraft paper which has been impregnated with a thermosetting resin and, more particularly, with a thermosetting water-soluble or water-insoluble phenolic resin, and a decorative sheet. When the kraft paper has been impregnated with the thermosetting resin, the sheets are dried and cut to the appropriate size. Thereupon, a plurality of these resin impregnated sheets are stacked in a superimposed relationship. The number of plies or sheets in the stack depends on the ultimate intended use of the laminate. For most purposes, the number of plies of these core sheets will total about six to nine but can total as many as 12-15. When particleboard is used, the material preferred is that produced from thermosetting resin impregnated wood chips which are heat and pressure consolidated into a composite structure. This particleboard is usually also referred to as flakeboard, chipboard, etc. and is well known in the art.
There is then placed on the stack of core sheets or particleboard, the decorative sheet which is generally a sheet of alpha-cellulose paper which bears a printed design or has a light color and is impregnated with a noble thermosetting resin which is not subject to significant darkening upon the application of heat. Suitable resins for the decorative sheets are the aminotriazine resins and more particularly the melamine-formaldehyde resins, the benzoguanamine-formaldehyde resins, the unsaturated polyester resins and the like. It is generally desirable, when making these decorative laminates, especially those produced with the kraft core sheets, to make use of a protective overlay sheet which is placed atop and is similar to the decorative sheet but is generally devoid of any design and, in the final laminate, is transparent. The superimposed laminate components are then inserted into a laminating press and are heat and pressure consolidated to a unitary structure. During the consolidation step, the thermosetting resins are converted to the thermoset state thereby providing an extremely hard, attractive and permanent laminated product. For obvious economic reasons, it is common practice, especially when producing the kraft paper supported laminates, to consolidate a plurality of these individual laminating assemblies into one large assembly, or press pack, said stacks being separated from one another by a release sheet, and then to subject this pack to heat and pressure consolidation.
In consolidating the laminate components according to the original, most widely practiced techniques, each individual assembly is placed with its decorative overlayment surface adjacent to a highly polished stainless steel press plate. The function of the press plate is twofold. First, it provides a smooth, defect-free surface to one side of the laminate. Second, in connection with the kraft paper based supported systems, it serves to separate pairs of back-to-back assemblies, thus permitting a plurality of these assemblies to be consolidated into laminates in one operation, usually in back-to-back relationship. In the art, the kraft paper base supported laminates are referred to as "high pressure" laminates, obviously because of the pressure used during the consolidation processes.
The surfaces of laminates produced in this manner are generally very glossy, and, as time passed, the consumer desired a less glossy surface. In the earliest days of the high pressure laminating art, the smooth, glossy surface produced during the pressing operation was reduced to a matte or less glossy finish by rubbing the surface with pumice, originally in an oil slurry, but more recently in a water slurry. For the purpose of this discussion such a flat, smooth, but reduced-in-gloss surface will be referred to as a "Class I" surface. A slightly textured surface can be produced by pressing the laminate surface against an aluminum foil caul stock, as is explained more fully below. Such a surface will be herein called a "Class II" surface and is described as "mini-textured" because the hilltop-to-valleybottom depths of such textures are from about 0.5 mil (0.0005 inch) to about 1.0 mil (0.001 inch). Somewhat coarser textures, sometimes called "low-relief" surfaces, can be produced by a printing process known as the "heavy ink method" such as is described in U.S. Pat. No. 3,373,068. These textures have hilltop-to-valleybottom depths of about 3 to 5 mils and will be herein called "Class III" surfaces. Finally, a very heavily textured surface can be produced by the methods of U.S. Pat. Nos. 3,860,470 and 3,718,496. The hilltop-to-valleybottom depths of these textures are of the order of 20 mils. These textures will be herein called "Class IV" surfaces.
To recapitulate the above:
______________________________________ Hilltop-to-Valley- Roughness bottom Depth Class Mode of Generation Micro Inches Mils ______________________________________ I Pumice Rub 8-35 Not Applicable II Aluminum Caul 50-120 0.5-1.0 Stock III Heavy Ink -- 3-5 IV "Substitute Caul" -- About 20 ______________________________________
The Class II surfaces have proven to be of special importance because they are attractive to the touch, yet serve to overcome "telegraphing" of joints, glue lines, coarse grain and other discontinuities which may occur in modern furniture structures, especially those which use frame construction as opposed to solid panels, e.g., in a table top. "Telegraphing" is used in the industry to describe and define the ability of a plastic sheet to reproduce in its upper surface whatever texture may be possessed by the substrate upon which it rests. Thus, for many years, the best practice in mounting high pressure laminates was to use smooth-surfaced, hardwood-faced plywood, usually birch or maple. With the advent of reconstructed wood particleboard for laminate substrates, it became the accepted practice to use three-layer construction which featured a smoothly sanded layer of "fines" on the bondable surfaces thereof to eliminate "telegraphing".
Furniture manufacturers learned that the Class II surfaces, described above, tolerate a much rougher substrate gluing surface and yet do not exhibit "telegraphing". This is because the Class II surfaces are not strictly flat, but have a slight texture which disguises the effects of "telegraphing" so that it is no longer noticeable to the viewer.
It should be understood that a Class II surface is only mildly textured and that the ultimate furniture user will not be unduly aware of the depth of the texture. For instance, such a textured laminate will provide a suitable writing surface if used as a desk top.
Class III and IV surfaces will also eliminate "telegraphing", but represent such departures from even approximate planarity that aesthetic requirements are violated and the utility expected of such approximately planar surfaces is absent.
Because of this unique dimension in texture, i.e., one which is coarse enough to hide "telegraphing", yet smooth enough to be accepted as planar, the "mini-textured" surface laminates have enjoyed great popularity and now account for over 50% of all the commercial laminates produced in the United States.
Earlier investigators used either embossed, machined or etched three dimensional metal press plates in making such decorative laminates, directly from these plates.
Embossed plates require the preparation of an embossing die or roll which in itself is expensive; but embossed patterns must be at least 0.002 inch in depth in order to compensate for wear on the die which will otherwise alter the character of the embossed pattern. Plates produced by machining are usually limited to geometrical patterns. The machining operation is slow and costly and not well suited to the preparation of a large quantity of plates; machined plates or dies are usually made in small numbers but are used to produce many pressings, usually in the millions. Etched plates are well known, but the cost of the etching baths, photographic equipment etc. requires an extensive capital outlay. Moreover, the process is critical and demands highly skilled artisans to carry it out. For the reasons mentioned, each of these processes is costly to begin with but may be even more costly from the standpoint of maintenance of the finished plate. These plates unavoidably become damaged through handling in normal use. Such damage is not due to any factor inherent in the process such as wear, corrosion, fatigue etc., but due to accidental events such as scratching, bending, burnishing and the like when the plate comes into undesired contact with other hard, sharp or abrasive objects. A small scratch only an inch or two in length can render useless an entire press plate of 5 feet .times. 12 feet dimensions. It is therefore of great importance that such plates be capable of easy repair. This is not true of embossed, machined, or etched plates which cannot be re-cycled through the process which originally produced them. The peening process described herein for producing the plates used in the instant invention, however, can be carried out repeatedly on the same plate to produce the identical texture at very low cost.
Other investigators proposed making the plates out of materials other than metal having the desired configuration but these were not satisfactory because of the failure, in one way or another, of these materials during the high temperatures and pressures used in laminating.
Aluminum foil bonded to paper, i.e., aluminum foil caul stock, especially in the production of the high pressure laminates, has been used, as is mentioned above, for about the last ten years as a texturizing medium in the production of textured laminates. This paper-foil combination is inserted between the overlay sheet and the press plate with the foil side adjacent to the overlay sheet as described by Ingram O. Robertson, Jr., "Use of Aluminum Foil Release Sheets in Decorative Laminates", TAPPI Plastics-Paper Conference, Chicago, 1971, or TAPPI Journal, Vol. 55, pages 1341-1344, 1972. Various textures can be generated in the laminates by choice of the finish on the foil and the texture of the paper used to back it.
Two of the particular combinations of aluminum foil and paper have become especially popular. Both combinations employ 0.0005 inch, matte finish, 1235 alloy aluminum foil. In one case, the foil is laminated to a 40 lb., machined-glazed natural paper. In the other case, a coated litho paper of about the same weight is laminated to the foil.
Unfortunately, aluminum foil caul stock is relatively expensive and during more recent years attempts have been made to find cheaper substitutes. One of the most common replacements employed is a glassine paper coated with a smooth layer of polymeric material containing a release agent. This material, although less costly than caul stock, produces laminates whose cleanability is poorer than those produced from caul stock and which are more prone to a surface irregularity common to both high and low pressure laminates called "mottle" which is caused by the paper used in the glassine release sheet. Apparently, caul stock successfully masks the structure of the paper to which it is bonded and thus reduces, although does not eliminate, mottle, while glassine does not. The glassine sheet is also much more flexible than the caul stock and hence often wrinkles during the laminate assembly operation, leading to rejected products. Furthermore, the glassine type of material also suffers because of the lack of uniformity in the gloss of the polymer release coating thus producing variable laminate surfaces.
Attempts to reuse caul stock after one pressing have proven futile because of the necessity for careful cleaning of the foil between uses and the rapid deterioration thereof due to creasing, etc. thereof. Attempts to use harder foil only reduced the onset of the deterioration.
A further investigation of press plates, in an attempt to eliminate the necessity for using caul stock, has also proven unsuccessful. Commercially available embossed steel plates were found to be wanting because the embossments thereon were too deep and shallower embossments could not be produced because the pattern on the embossing roll used to emboss the plates would not wear well enough to produce sufficient plates to make the operation viable.
A commercially successful flame-spraying technique which would result in press plates of the proper roughness and gloss required to produce the ultimate laminates has also been considered. In this process, stainless steel plates were coated with metal and metal oxides by flame spraying to yield plates covered with thousands of small, spherical particles and resembling sandpaper in appearance. The plates appeared to be satisfactory with regard to roughness but when modification of the gloss was attempted, they could not be made uniform on both the hilltops and the valleys. Laminates produced therefrom were of poor surface quality.